The composite skin assemblies of many of today's aircraft models utilize a carbon fiber skin mated to a metal or composite sub structure. In the case of an aircraft wing assembly process, the skin is shimmed to the structure to ensure a solid mate using a combination of metal, glass and liquid shims. Holes are either pre-drilled or match drilled through all layers so that various types of fasteners can be used to permanently assemble the structure. Various fasteners are used throughout the wing structure.
In a typical aircraft manufacturing process, thousands of specialized bolts and other fastener types are used for fastening composite or aluminum surface skins onto respective ribs and spars. The use of the correct size and length bolt is critical to ensuring that the bolt grip is fully engaged on the structure, and that the bolt can still be fastened to achieve the proper pre-load. It is also important to ensure the bolts installed are not too long as this adds to the overall aircraft weight (and hence fuel efficiency). Most manufacturers today install bolts using a manual process.
In the current manual process, the only means to locate holes is using a copy of the corresponding engineering drawings. The operator must reference the drawings to locate features on the wing and count holes from a reference point to arrive at the intended hole for the fastener. There are numerous sources of potential errors in this process. The operator could miscount holes, misidentify drawing features, or simply mistakenly choose the wrong hole. The requirement to have a complete set of hard-copy drawings available is both difficult to control when full production rates are underway, and it requires space to read the drawings.
Many fastener assembly steps require an operator to perform multiple functions before completing a fastener assembly. When measuring the grip length, the operator must gauge the hole and record the result with a grease pencil. When installing fasteners, the operator must look up which fastener is appropriate for the hole and retrieve it. During all of these steps, it can become time-consuming to constantly refer to the engineering drawings to locate the working hole. All of this time adds no value to the assembly of the wing and introduces many opportunities for error.
The proper bolt grip length for every hole on the wing is determined by measuring the composite thickness of the material at the hole. A manual grip gauge instrumented with a digital Vernier scale is currently used to acquire the thickness measurement. The resulting material thickness is recorded manually using a grease pencil on the wing beside the measured hole. The amount of manual operations and the fragility of the final data make this process potentially prone to bolt selection errors. For example, the grease pencil recordings could be damaged or misinterpreted causing an operator to choose the wrong bolt length. In an effort to speed up the measurement process, operators could become more vulnerable to recording incorrect results. The location of the grease pencil record could also lead to problems later in the assembly process. For example, with the holes on the wing being close together, the operators could use the wrong value for the current hole. Furthermore, the grease pencil is not a permanent method of recording results and is susceptible to smudging or accidental removal. Some assembly processes like the upper wing skins require the measurement process to occur at a different assembly bay than the fastener installation. During transport of the wing assembly, the recorded values are vulnerable to damage or removal. Any amount of destruction in the recorded grease pencil marks will add time to the assembly process as the holes would have to be re-measured. If the result was recorded in another location (i.e., on the engineering drawings) then this value would have to be found and re-recorded in the proper location. In the event that no other record of the measurement exists, then the measurement process would have to be repeated. The problems identified can lead to installing an incorrect fastener grip length. The grease pencil further increases non-value added time to the process for cleaning and removing the residue after the results are no longer needed.
In addition to the physical inefficiencies of measuring and recording the skin thickness, data collection issues also exist. The current process has no capability to archive and track process critical information. A requirement for ongoing process improvement is the ability to store such data for analysis. Such analysis can lead to possible assembly problems that may have otherwise gone unnoticed, or show trends in certain areas of the process that can be manipulated to increase throughput. Without proper storage of data, these benefits cannot be realized.
A complicated step in the installation of fasteners is the final selection of the correct grip length. Engineers have developed some spreadsheet tools to assist in the selection of fastener components. Due to the complexity of these tools, an engineer is required to be used during the assembly process to properly interpret the data. After entering this information into a spreadsheet, the engineer must then analyze the results for acceptable thread protrusion and to avoid a shank out condition.
Further complexity is introduced in some areas where removable skin panels are being installed. Rather than measuring the skin thickness to determine the bolt grip length, the tools are used to determine the proper nutplate size to install. This requirement allows the removable panels to be installed with the same fastener diameter and grip length for ease of maintenance.
The time spent selecting the fastener components is very inefficient due to the complexity involved. This problem is compounded by the fact that an engineer is being used during this process step. Engineering resources can be utilized in a more efficient manor. The current system is both prone to error in using the look-up tables, and inefficient in the use of operators and engineers.
Once the fastener has been properly identified, the operator must then retrieve it from a storage cabinet. Significant increases in cycle time are experienced from the operator having to make numerous trips to the cabinet to retrieve fasteners. The potential for error exists in this step of the process as the operator must choose the correct fastener and place it in the proper hole. Visually identifying that the fastener is the correct grip length is very difficult when the increments are 1/16″ or 1/32″, so a bolt length gauge must be used to confirm the bolt length. If the fastener storage units are not well isolated, the chance for mixing incorrect fasteners into a bin could result in the wrong fastener being installed. Even with very strict process regulations for filling storage bins in place, the fasteners may be delivered from the supplier with incorrect fasteners in a batch. The current process has no efficient means of ensuring the correct fastener is selected every time.
These issues can also increase the risk of foreign object damage (“FOD”) reaching the aircraft. For example, if a fastener is threaded into a click-bond nutplate and is too long, the nutplate could become dislodged and fall into the fuel tank. In addition, if operators are required to carry a sample of fasteners with them during installation, they run the risk of dropping fasteners into work areas accidentally. Elimination of the risk of FOD is essential due to the damage that could be caused as a result.
A critical step in the wing assembly includes the application of sealant and its promoter to the bolt to seal the fuel tank sections. Many holes in the wing skin are in areas on the fuel tank and must be properly sealed to prevent leaks. Ensuring proper adhesion of the skin sealant requires a two step process with two time critical stages. The first step involves pre-coating the fastener with an adhesion promoter. This substance is essentially a catalyst to the final sealant when the two are combined. The current method of applying the promoter is a simple immersion of the fastener in a container of the promoter material. This step is slow, and does not produce repeatable, efficient results. It could also lead to outside contaminants adversely affecting the sealing. The promoter must be applied to the fastener and allowed to air dry. The length of time required for air drying depends on the type of promoter and sealant being used on the fastener. A widely used sealant is PR-2001 available from PRC DeSoto whose promoter must be applied and air dried for 30 minutes before the sealant can be applied. There is also a maximum time permitted to elapse before the sealant is applied. If the maximum time is exceeded, the promoter must be reapplied, and the time constraints are repeated. The second step in the sealing process involves dispensing the sealant onto the fastener. It is critical that the sealant only be applied locally to the underside of the fastener head and the shank. Sealant must not come in contact with the threaded portion of the fastener. The dispense operation is currently carried out by an operator using a mixing style syringe. The amount of sealant dispensed is completely operator dependent. The volume and bead size will vary on every hole and even more with different operators performing the application. This process can lead to a great deal of material “give-away,” meaning a considerable amount of sealant is being needlessly applied due to the inconsistent nature of the application. The volumes used from one fastener to the next, and one operator to the next may be relatively small. However, when this is translated to thousands of holes on a wing, and thousands of wings being produced, there becomes a large potential for savings. The current process of applying the adhesion promoter and sealant does not have sufficient control over logging and monitoring any time criteria. Without such controls in place, there is a risk of repeating work unnecessarily.
The process of physically installing the fasteners is tied closely with the application of sealant as previously described. The sequence of fastener installation is critical to ensure that the faying sealant used between the wing skin and substructure does not cure before the skin is tightened down to its final position. For example, during the build of certain upper wing portions, engineers have identified numerous fastener locations that must be installed first to help smooth out the wing skin. For example, these fasteners must be installed in a specific order and tightened, e.g., to 80% of their final torque specification. After this is complete, then the operator must repeat the sequence to finish the installation at 100% full torque.
The next step in the current process is to identify and install fasteners in a specific sequence, e.g., every 4th hole of the wing skin. These fasteners will again be installed to 80% of final torque, and then the process repeats to tighten them to 100% full torque. This entire procedure must be completed within the span time of the faying sealant. This process introduces a number of inefficiencies due to the segmented steps and torque requirements. It is very time-consuming to locate holes on the wing surface. This issue is evident again during fastener installation because the holes are not done sequentially based on location. The torque procedure that is required also leads to added setup time to ensure the proper torque parameters are being used at all times. The torque specification must be looked up on an engineering drawing for each hole, and must then be configured properly in the torque controller. This process is repeated twice to accommodate the initial installation of fasteners to 80% full torque, followed by the final installation.
The current process outlined is very prone to errors as the number of fasteners in the wing increases. With a lack of sufficient control over the status of every fastener, it is possible for the operators to lose track of which fasteners belong in the current install procedure, as well as which have been installed to 80% torque versus fully installed. This confusion could lead to fasteners being installed incorrectly.
Thus, there has been a long felt need for new and improved fastener automation systems and method of assembly that overcome at least one of the aforementioned problems.